Use of EMFi as a blood pressure pulse transducer

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Use of EMFi as a Blood Pressure Pulse Transducer Hannu Sorvoja, Vuokko-Marjut Kokko, Risto Myllylä, Member, IEEE, and Jari Miettinen

Abstract—This paper describes and tests two prototype series of pressure transducer arrays based on electromechanical T resistance, EMFi is film (EMFi). By offering high an excellent material for low-current long-term measurement applications. About 50 transducer arrays were designed and tested using different configurations and electrode materials to sense low-frequency pressure pulsations on the radial artery in the wrist. Essential requirements included an adequate linear response in the desired temperature range and uniform quality. Transducer sensitivity was tested as a function of temperature in the range of 25 C–45 C at varying dc and ac pressures. The average sensitivity of the EMFi used in the transducers proved adequate ( 2.2 mV/mmHg and 7 mV/mmHg for normal and high-sensitive films) for the intended purpose. Moreover, EMFi’s spectral response covered the required range for biomedical applications, but it was unable to measure static pressure 38 Hz . The sensitivity of the EMFi material was 3 dB sufficiently constant for measuring blood pressure pulses in the desired range (0–300 mmHg), and the best achieved deviation in sensitivity was 5.1%. It was also established that in addition to sensitivity and its standard deviation, crosstalk between electrode elements also depends strongly on electrode thickness.





Index Terms—Blood pressure monitoring, noninvasive, pressure transducer array, pulse transit time, pulse wave velocity, radial artery pulsation.



LECTROMECHANICAL FILM (EMFi), previously known as electrothermomechanical film (ETMF), has a thin porous polypropylene film structure. The film is produced by injecting gas bubbles into a molten plastic and then extracting a tube of this gas–plastic mixture with spherical bubbles. The tube is expanded into a thin film by blowing, which produces biaxially oriented bubbles in it. Next, the film is stretched, which transforms the bubbles into flat discs with a lateral dimension of 10–100 m and a vertical dimension of about 3 m. The final thickness of the film is 37–70 m depending on the type of processing it has undergone. After that, the film is permanently charged either by a plane electrode corona discharge system in a high electric field or by using electron beam charging. To provide electrodes, EMFi is metallized on both sides using one of three methods: sputtering, vacuum evaporation, or gluing a metallized thin polyester film, for instance. In addition to the material itself, the existing patent also covers the manufacturing method [1]–[3].

Manuscript received June 1, 2004; revised January 31, 2005. This work was supported by Polar Electro Oy. H. Sorvoja, V.-M. Kokko, and R. Myllylä are with the Department of Electrical Engineering, Optoelectronics, and Measurement Techniques Laboratory, University of Oulu and Infotech Oulu, FIN-90014, Finland (e-mail: hannu. [email protected]). J. Miettinen is with the Polar Electro Oy, 90440 Kempele, Finland. Digital Object Identifier 10.1109/TIM.2005.853345

The operation of the transducer is of capacitive nature: when an external force or pressure introduces mechanical compression into the film, the spatial distribution of charges within the material changes with respect to the electrode’s layers. As a result, a mirror charge proportional to the force is induced at the electrodes. This charge can be measured with a charge amplifier or a voltage amplifier with very high input impedance. Biomedical field tests on EMFi include detecting motor activity and breathing movements in laboratory animals [4], [5], but it has also been used to measure respiration movements and blood pressure pulsations in humans [6], [7]. Furthermore, the material has been used in smart houses to monitor demented elderly people [8], to recognize walkers on an EMFi floor [9], and to measure other physiological signals [10]. The film can also be used as an actuator or a loudspeaker, because it undergoes thickness alterations, when a high voltage is applied between the electrodes [11]–[19]. With a useful spectrum extending as far as 500 kHz, it also serves as an ultrasonic transducer [19]. There are two different types of film, standard and highly sensitive, which have a sensitivity of about 40 and 200 pC/N, respectively. This sensitivity is strongly dependent on the thermal environment during processing, storage, and operation time. Thus, the charge tends to decrease, if the temperature remains at over 50 C for a long period of time. This characteristic of the material limits its application range, but the material can also be aged to stabilize its sensitivity. The properties of EMFi, including sensitivity and manufacturing process, have been widely researched and published in recent years [20]–[37]. The results indicate that sensitivity can be enhanced by means of different gas atmospheres and pressures during corona charging and by using different gases for bubble filling. EMFi’s temperature range can be extended by developing the manufacturing processes or changing the base material. As a result, EMFi is a highly interesting alternative for applications that call for a very sensitive and inexpensive transducer material that only requires a simple amplifier. Due to the relatively large gas voids and local corona breakdowns, sensitivity may vary in different parts of the film. This may lead to problems when an application presupposes the use of a very small transducer. A case in point is a pressure transducer measuring blood pressure pulsations over the radial artery in the wrist. In this application, the transducer must be of the array type, because the correct pulse pressure waveform is obtained exactly from a single element sitting over the artery. Signals produced by edge elements may be decayed and noisy due to motion artifacts. Array transducers based on EMFi have been produced, tested, and used to measure the heartbeat rate of immobile and moving persons [38], [39]. In addition, they have been utilized in noninvasive blood pressure measurements on healthy volunteers and

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cardiac patients. In these measurements, a cuff was applied to the upper arm, and pulsations were sensed on the distal side of the wrist. Essential findings have already been published in several conference proceedings [40]–[44] and journals [45] and [46]. In blood pressure measurements of this kind, based on the electronic palpation of the amplitude and the transit time of pressure pulse waves, pulsations are detected by a wrist array transducer during the inflation or deflation of the cuff. In the deflation mode, cuff pressure is rapidly inflated over the presumed systolic pressure level and then slowly deflated under the diastolic level. The onset of pulsations corresponds to systolic blood pressure. Diastolic blood pressure, on the other hand, can be determined by the point where the delay in the pressure pulse sensed by the transducer array stops to diminish [47], [48]. The same method can be used both in the deflating and in inflating cuff pressure mode, but the deflation mode is preferable. If the transducer array is dense enough to contain four elements per millimeter, it can used as a tonometer type of blood pressure transducer array. In this array, the transducer element that is located over the central part of the artery measures interior pressure with a high degree of accuracy, dependent mostly on the hold-down pressure and the accuracy of each transducer array element. In addition, this type of measurement also requires that the transducer’s bandwidth starts at dc [49]. This paper demonstrates some essential characteristics of EMFi used as a miniature array transducer in biomedical applications. Such transducer arrays, connected to a simple amplifier, were used in a number of measurements, partly for the purpose of investigating EMFi as a transducer material, and partly to conduct actual measurements on cardiac patients and healthy volunteers in a moving and nonmoving state. The results show that, as far as physiological measurements are concerned, EMFi transducers are both practicable and durable, which is also attested by the fact that a number of them were used for an entire year. Certain aspects concerning the use of EMFi in measuring blood pressure signals are patented by the European Patent Office [50]. II. METHODS This study made several modifications to existing practices to construct small-sized transducer arrays. Commonly, transducers are made separately and connected to an amplifier unit with lengthy transfer lines made of Kapton printed circuit board (PCB). Such transfer lines, however, increase input capacitance, which serves to decrease the amplified signal. In the experiments reported here, the transducer and the transfer lines to the amplifier were both made of flexible Kapton PCB. The equivalent circuit for EMFi consists of a charge generator in parallel with a leakage resistor and a capacitor. As transfer line capacitance and amplifier input capacitance are also parallel, the amplifiers must be placed as close to the transducers as possible. A second modification involves exchanging charge amplifiers for CMOS voltage followers with high input impedance. These amplifiers are simple and do not take up much room. The transducer array was made by gluing a Kapton plate on a plastic base component using a double-sided adhesive produced by 3M. This

Fig. 1. Eight-element transducer array with EMFi as pressure sensing material.

plate served as the ground plane and the second electrode for the transducer. Next, the EMFi material was glued on top of this plate using the same tape. The following, similarly attached, layer comprised a ground sealed eight-element transducer electrode plate (Kapton PCB). The dimension of the sensitive area of the transducer arrays was approximately 7 11 mm, as seen in Fig. 1, which shows the device in its natural size from three different angles. Transfer lines were soldered to a standard PCB containing the amplifiers. This configuration has a transfer line capacitance of 10 pF, paralleling that of the transducer elements, and its leakage resistance lies in the T range. Such transducer arrays were used to study the sensitivity of sensors both as a function of temperature and of ac and dc pressure amplitude. The pyroelectrical behavior of EMFi manifests itself in offset voltage drift. This offset voltage is affected by the permanent charge of the EMFi material loaded in the transducer, the leak resistance of the transfer lines and electrodes, and the input bias currents of the operational amplifier. Fig. 2(a) presents an ideal operational amplifier with electrostatic discharge (ESD) diodes in its input, which is common practice for amplifiers with high input impedance. A mismatch in the properties of these diodes causes an input bias current. If the diodes are exactly similar, a current is generated only from the positive to the negative power supply pins, and does not affect the input bias current and offset voltage. A mismatch, on the other hand, produces a potential that deviates from zero. Fig. 2(b) presents these currents as a function of potential at two different temperatures. When the temperature rises, the offset voltage of the unconnected amplifier can be located at the crossing point of the current curves. Thus, a change in temperature can produce an offset voltage drift. When the transducer applies a very low frequency input ), the bias current increases. The bias cursignal (equal to rent to and from the transducer equals the difference between . In addition, elevated these curves at the voltage value temperature increases the bias current and may decrease the amplitude of the low-frequency signal: the operational amplifier draws charges to itself. Another important consideration relates to the sensitivity of the different elements. Quality variations in the EMFi material and potential gluing faults may produce sensitivity variations. Such variations are also produced during storage, particularly by alterations in temperature. Other important factors in this respect include pressure distribution and crosstalk between different channels (array elements). Fig. 3(a) shows the transducer



Fig. 2.

(a) ESD diodes in the input pins of the operational amplifier and (b) their influence on bias currents at two different temperatures.

Fig. 3.

Improved version of the EMFi transducer array used to study sensitivity and crosstalk between different elements. (b) is magnification of (a).

array, while Fig. 3(b) displays the same array in magnification. The entire sensor is well shielded to avoid noise. Instead of soldering, the ground electrode can be glued with an electrical connective glue to avoid introducing a temperature shock in the EMFi. As the sensor is not in direct contact with the skin, arterial pressure is mediated to it by means of plastic heads of exactly the same size as the transducer element, i.e., 2 0.6 mm with a 0.4-mm gap between them. Two kinds of pressure generation modules were developed to measure the responses of the transducer arrays. The first one comprised a computer-controlled electromagnetic pump capable of producing a pressure pulse with a preprogrammed wave shape. The system was quite complicated; a PCB (sitting in a personal computer) was used to drive power circuits in an outside module feeding an electromagnetic membrane pump. This pump was attached to a metallic chamber with a plastic cover. The cover contained a cavity, which fitted exactly the transducer array described in Fig. 1. The chamber was filled with water, and a silicon rubber was used to isolate the water from the transducer array (see Fig. 4). In addition, the chamber housed a pressure transducer (Sen Sym PS 15GC), whose signal served as feedback signal to the PCB. After a few pulses, the system adapted to the desired signal and produced exactly the desired waveform. This measurement used only a sinusoidal 1-Hz signal. As this pressure generation system was incapable of producing high spectral signals, another pressure generator had to be used for this purpose. The other generator, consisting of a

Fig. 4. Transducer array mounted on the pressure chamber, which produces a sinusoidal 1-Hz signal using a membrane pump. The membrane pump was controlled by a computer system. Also, temperature inside the chamber was controllable.

piezoelectric actuator (Physic Instrument P-840.60), was used to generate sinusoidal signals for one transducer array element at a time. Contact pressure was measured from a piezoelectric actuator by means of a force transducer (Honeywell Micro Switch FSG-15N1A). The size of the contact head equaled that 2.0 mm, which enabled the of the transducer element, 0.6 investigation of crosstalk between the different transducer elements. In every measurement, a personal computer with a National Instruments data acquisition board (AT-MIO16 or DaqCard-700) was used for file readouts. The measurement system is illustrated in Figs. 5 and 6.



Fig. 7. Pressure step response of the EMFi transducer using TLC2272 as the operational amplifier at a room temperature of 21 C. Fig. 5. Measurement arrangement for measuring the second set of transducer arrays. The force sensor detects the sinusoidal force generated by the piezoactuator. Both this and the signal from the EMFi transducer are amplified and then recorded by a personal computer (PC) using National Instrument’s data acquisition board.

Fig. 8. Offset voltages of the eight-element transducer array as a function of time with eight temperature steps.

Fig. 6. Magnified picture of the aligning system presented in Fig. 5 to measure the sensitivity of the EMFi material. A micropositioning translation stage is used to move the test head just above the sensor element under test. The test head is aligned exactly on top of the element.

III. RESULTS AND DISCUSSION First, the step response of the transducer was measured using a pressure chamber at room temperature (22 C). Fig. 7 presents the response signal with the time constant of 1 h and 10 min. In this test, the sensors covered an area of 5 mm , and the operational amplifier was TLC2272. Since the time constant yields 38 Hz as the lower limiting frequency, the sensor material can be considered as suitable for biomedical applications. It must be noted, however, that the time constant is strongly dependent on temperature and the amplifier’s input bias current. Second, the temperature behavior of the transducer was studied by placing a sensor in a temperature cabin, while the temperature was decreased from 40 C to 0 C by 5 C steps. Fig. 8 shows the offset voltages as a function of time. As can be seen, EMFi exhibits pyroelectrical behavior. The film produces upward spikes, but the bias currents drive the voltage down. Consequently, when a sensor based on EMFi is employed to

Fig. 9. Offset voltages of the EMFi transducer array elements as a function of temperature.

detect sub-Hertz pressure alterations, temperature should be maintained at a stable level near room or body temperature. This pyroelectric behavior was further studied by decreasing the temperature linearly during a one-week period. The thus obtained offset voltages are presented in Fig. 9. As the figure shows, the offset voltage is almost zero between 25 C and 40 C. This indicates that the sensor material is suitable for biomedical applications in which the sensor is either very near


Fig. 10. Offset voltages as a function of temperature when different voltages are applied to the “ground electrode” of the EMFi transducer and output pins are reset with different voltages.

Fig. 11. Offset voltages as a function of temperature when different voltages are applied to the electrodes of a capacitor and reset.

or in contact with the skin and, thus, near the body temperature. Nevertheless, when the sensor is in actual contact with the skin, problems may arise due to humidity evaporating from the skin. Because the transducer is capacitive by nature, its electrodes must be properly sealed to prevent humidity from affecting the edges of EMFi’s electrode interface. Since the characteristics of the operational amplifier evidently affected the measurements, the EMFi material was tested by resetting the amplifier’s input electrode to a positive or negative supply voltage and taking a readout when the voltage was stabilized. Also, the other electrode was connected to the positive or negative supply voltage instead of zero voltage as usual. The produced by the other electrode is influence of the voltage clearly indicated by several readings at different temperatures in Fig. 10, whereas the influence of the reset voltage was very small. In a further test, the transducer was replaced with a 10-pF polystyrene capacitor (see Fig. 11). As the leakage resistance of the capacitor roughly equals that of the EMFi material, Fig. 11 presents only the behavior of the operational amplifier. The test shows that the EMFi measurement differs considerably from the capacitor measurement. The characteristics of EMFi seem to dominate over those of the amplifier at higher temperatures. According to Figs. 10 and 11, the offset voltage can be compensated for by applying opposite polarity voltage to the “ground”


Fig. 12.

Sensitivity of the transducer array as a function of temperature.

electrode of the EMFi transducer by means of an integrator, for example. The sensitivity of the transducer array was measured with a pressure pump chamber. Fig. 12 presents the sensitivities of the eight elements as a function of temperature using a 1-Hz pressure signal with a sinusoidal shape. Although the sensitivity of the various elements varied considerably, this variation appeared to be independent of temperature. Next, a set of similar measurements was carried out, but this time sensitivity was measured as a function of pulse pressure (ac) and constant pressure (dc). In these measurements, ac pressure describes the difference in the pulse pressure amplitude from the highest to the lowest value and dc pressure refers to the mean value. In the ac measurements, dc pressure was 100 mmHg and ac varied from 10–180 mmHg in 10-mmHg steps. In the dc measurement, on the other hand, ac pressure was 100 mmHg and dc pressure ranged from 10–130 mmHg. Being almost identical to those in Fig. 12, the results will not be presented here. The average sensitivity of the six transducer arrays that were involved in these measurements was 6.4 mV/mmHg 18%. Variation in the sensitivity of the different elements in each array was smaller (5% to 10%). A second set of transducer arrays was studied by investigating crosstalk and quality (sensitivity deviation) in different parts of a high-sensitivity EMFi sheet. To stabilize its quality, the sheet was first aged by the manufacturer. Then the sheet, measuring 60 85 cm, was divided into 4 5 equal parts. Little sections were then cut out of selected parts in the horizontal and vertical direction to be used as sensors. The transducer arrays were made separately for each PCB and measured at the 1-Hz sinusoidal frequency. Every measurement was made four times and then averaged. The results of this set, totaling 800 measurements, are presented in Table I. Due to aging, the sensitivity of this transducer array set was much lower than that of the previous set, particularly around the edges of the sheet. Furthermore, the standard deviation of this set was quite high, indicating that the transducer material is potentially inadequate for certain measurements, such as measuring the heart rate of a jogger [38], [39]. Sensitivity was also analyzed as a function of dc and ac pressure. Compared with the previous set, this set used considerably higher pressure values. Presented in Figs. 13 and 14, the results point out that, in contrast to the previous set, sensitivity




Fig. 15. types.

Average values and standard deviations of all EMFi transducer array

Fig. 16.

Average crosstalk values of all EMFi transducer array types.

Fig. 13. Sensitivity of the eight-element transducer array as a function of dc pressure at an ac pressure value of 300 mmHg.

Fig. 14. Sensitivity of the eight-element transducer array as a function of ac pressure at a dc pressure level of 700 mmHg.

decreased when dc pressure increased. On the other hand, the transducers’ sensitivity to ac pressure was stable, which is consistent with the result of the previous set. The standard deviation of sensitivity measurements can be improved by laminating two or more films on top of one another. Hence, five couples of transducer arrays were made by laminating two EMFi layers. This method produced only a slight improvement in sensitivity, but had a marked effect on the sensitivity deviation between the elements, resulting in 2.62 mV/mmHg 7.7%. To measure crosstalk, two couples consisting of two types of transducer array were made by adhering a thick (200- m) or a thin (70- m) aluminum film on an EMFi film, functioning as a ground electrode. Figs. 15 and 16 show the average values for the sensitivity, deviation, and crosstalk of all these transducer types, including also single and sandwich-type transducer ar-

rays. As can be seen, with 3.34 mV/mmHg 5.1%, thin aluminum produces the best sensitivity and lowest standard deviation and crosstalk values. Fig. 17 presents typical results of a real blood pressure measurement using increasing cuff pressure. Pressure pulsations in the radial artery are detected with a four-element transducer with the strongest pulsations in the third channel. When cuff pressure exceeds the diastolic blood pressure, pulsations start to be delayed and their amplitude begins to diminish to disappear altogether on exceeding the systolic blood pressure level. Consequently, diastolic blood pressure can be determined on the basis of either the amplitude decrease or the change in pulse transit time. Of these, the latter method is preferable, because pressure pulse reflections from the finger side produce alterations in the amplitude of the pulse. When pressure in the cuff exceeds the diastolic pressure, the pressure pulse detected by the transducer array will be constantly delayed until it disappears completely. The same method, first presented by Geddes et al. [47], [48] and later by Sorvoja et al. [42], [45], and [46]can also be used with deflating cuff pressure. In addition to heart rate and blood pressure measurements, the transducer array can be applied to measurements of pulse wave velocity (PWV) to assess vascular stiffness produced by atherosclerosis, one of the most common vascular diseases [51]. In PWV measurements, each pressure pulse is measured in two locations and PWV is calculated from the distance between them



PanPhonics Oy. The authors would also like to thank Screentec Oy for the advice that they provided during the manufacture of the transducers. REFERENCES

Fig. 17. Blood pressure measurement with inflating cuff pressure. The transducer array has four elements whose signals are filtered at the 1.7–11-Hz bandwidth. Intra-arterial blood pressure and cuff pressure are in the middle and ECG on the bottom.

divided by the pulse time delay. Coupled with pulse shape analysis, PWV measurements can also be used to calculate vascular resistance and perhaps even cardiac output. Shape analysis of this kind requires a wider measuring bandwidth, starting from about 0.1 Hz and extending to 20 Hz. IV. CONCLUSION This paper discussed the use of EMFi in the making of miniature pressure transducer arrays to sense pulsations on the radial artery in the wrist. The paper also described the process of developing two different types of array; in the first type, the array’s ground electrode is in direct contact with skin, and in the second, pressure is transmitted to each element through a plastic plate. The influence of the transducer amplifiers’ bias current was considered separately from the behavior of the sensor. These measurements involved a large pressure scale, 0–2700 mmHg, while temperature behavior was studied in the 0 C–50 C range. The quality of the EMFi material was also studied statistically in different configurations. The results show that the material can be successfully used, provided that the temperature and sensitivity deviation demands are not too high. Although the quality of the material is not sufficiently uniform to be applied to tonometer blood pressure measurements, it can be used in measurements of blood pressure, heart and breath rate, pulse shape, and pulse wave velocity. In practical use, EMFi transducer arrays were found to be both durable and reliable during two large measurement sets conducted in the hospital environment. One of the best advantages of EMFi is its low power consumption: all that is required is voltage followers with high input impedance. Another advantage is low price; EMFi can be manufactured by standard plastic packing machines with corona charging, which serves to keep the price down. ACKNOWLEDGMENT The authors would like to thank Emfit, Ltd. for delivering the EMFi material used. EMFi is a trademark of Emfitec and

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Hannu Sorvoja was born in Oulainen, Finland, in 1966. He received the Master’s and Licentiate degrees in electrical engineering from the University of Oulu, Oulu, Finland, in 1993 and 1998, respectively. He is currently a Senior Assistant in the Department of Electrical Engineering, University of Oulu. He is involved in many research and application projects as a Researcher and Project Leader. His present activities are focused on development of noninvasive low-power wireless biomedical measurement methods and sensors and implementing them into a hospital environment.

Vuokko-Marjut Kokko was born in Oulunsalo, Finland, on July 6, 1965. She received the M.Sc. degree in electrical engineering from the University of Oulu, Oulu, Finland, in 1999. In 1999, she joined the Optoelectronics and Measurement Techniques Laboratory, University of Oulu, as a Researcher on a research project aimed at the use of EMFi as a small-scale pressure transducer.

Risto Myllylä (M’97) was born in Haapavesi, Finland, in 1945. He received the D.Sc. (Eng.) degree in electrical engineering from the University of Oulu, Oulu, Finland, in 1976. He has been an Associate Professor in the Department of Electrical Engineering, University of Oulu, since 1974 and Professor since 1995. From 1977 to 1978, he was a Visiting Scientist at the University of Stuttgart, Stuttgart, Germany. From 1988 to 1995, he was a Research Professor at the Technical Research Centre of Finland. His research interests include industrial and biomedical instrumentation development, particularly in optical measurements. He has coauthored one book and over 330 papers and patents. Dr. Myllylä is a Past President of the Finnish Optical Society and Advisory Committee Member of the European Optical Society (EOS). He is a Member of SPIE, OSA, and EOS.

Jari Miettinen received the Diploma Engineer degree from the University of Oulu, Oulu, Finland, in 1995. His professional interests include ambulatory biomedical instrumentation, especially heart rate measurement with novel sensors and development of signal processing. Since 1998, he has been with the Polar Electro Oy, Kempele, Finland. He is the Manager of the technical research team, and his group is involved in development of low-power circuit technology and applications.

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